The Nervous System
Overview
the ultimate control center of the body overseeing all communication among the organ system
Sensory Input: receives stimuli via millions of sensory receptors throughout the body
Integration: processes the input stimuli and decides what should be done
Motor Output: activates effector organs to cause a response
the nervous system process stimuli and tells effector organ how to react
Composed of nervous tissue which is densely packed with:
Neurons: (nerve cells) = excitable cells that respond to stimuli by conducting impulses to transmit signals, 10% or nervous tissue
longest living cells in body (with proper care and nutrition, it can last a lifetime)
irreplaceable
Amniotic: one specialized, they can no longer go through cell division if damaged
They require SO MUCH energy due to high metabolic rate
Need constant supply of GLUCOSE and OXYGEN
About ¼ of daily food intake goes to neuron activity
Bundles of axon = tracts in CNS and nerves in peripheral NS
Neuroglia: (glial cells) = supportive cells that provide nutrition, insulation, and help with signal transmission, 90% of nervous tissue (6 types)
Astrocytes (CNS): maintain blood brain barrier, provide structural support; regulate ion, nutrients, and dissolved gas concentrations; absorb and recycle neurotransmitters; form scar and tissue after injury
Ependymal Cells (CNS): line ventricles (brain) and central canal (spinal cord); assist in producing, circulating, and monitoring cerebrospinal fluid
Microglial Cells (CNS): remove cell debris, wastes, and pathogens by PHAGOCYTOSIS
Oligodendrocytes (CNS): myelinate CNS axons; provide structural framework
Satellite Cells (PNS): surround neuron cell bodies in ganglia; regulate oxygen, carbon dioxide, nutrient and neurotransmitter levels around neurons in ganglia
Schwann Cells (PNS): surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after energy
Neuron Structure
Soma: (cell body) = the life support containing the nucleus and most organelles (tons of mitochondria)
Ganglion: collection of nerve cell bodies located in the body (not brain or spinal cord)
Nerves: bundles of axons that extend from the brain and spinal cord to the rest of the body
Processes: extensions from the cell body
Dendrites: the main receptor of signals; input region
Axon: generates and transmits nerve impulses; conducting region/nerve fiber
Axon Terminals: the end of the axon that releases neurotransmitters at a synapse when a nerve impulse is received; secretory region
Myelin Sheath: covers long axons/nerve fibers to protect and electrically insulate them to increase the speed of nerve impulse transmission
Nodes of Ranvier: unmyelinated gaps in the myelin sheath that aid in increasing the velocity of nerve signal conduction
Classification
Neurons can be classified by their structural differences
Based on their number of processes (extensions) from the cell body
Multipolar: meaning more than 3 processes (1 axon and 2 or more dendrites)
99% of neurons are multipolar
Bipolar: 2 processes (1 axon and 1 dendrite on opposite ends of the cell)
Rare, found in a few special sense organs
Unipolar: 1 process, dividing from the cell body like a “T”
in the ganglia (group of sensory neurons) in the PNS
Neurons can be classified by their functional differences
Based on the way an impulse travels through a neuron with regards to the brain and spine
Sensory Neurons (afferent neurons): transmit info from sensory receptors to CNS, most are structurally unipolar
Motor Neurons (efferent neurons): transport info from CNS to rest of the body, most are structurally multipolar to send impulses to multiple places
Interneurons (association neurons): housed in the CNS and transport info between the sensory and motor neurons, therefore most are structurally multipolar
Central Nervous System (CNS)
brain and spinal cord
Brain
uses 20% of oxygen in body
25% of glucose powers brain
approximately 3.1 lbs (adult)
83% of brain is the cerebral hemispheres (cerebrum)
Right hemisphere: controls left side of body; deals with creativity, artistic, imagination, intuition, musical, and spatial abilities
Left hemisphere: controls right side of body; deals with speech, logic, sequencing, comprehension, math and writing
Spinal Cord
spinal nerves carry impulses to and from the spinal cord
integration and control center
the brain is protected by the skill and surrounded by layers of tissue (meninges) and cerebrospinal fluid that cushion the brain from injury
Ventricles: hollow fluid-filled cavities within brain that contain the choroid plexus which makes cerebrospinal fluid
3 main parts of the brain
Cerebrum: the largest part of the brain
made of left and right hemispheres
divided into 4 lobes
function in learning, speech, emotion, reasoning, vision, hearing, and fine movements
Surface is the cerebral cortex and is arranged in folds to increase surface area
Cerebellum: under cerebrum
maintains posture and balance
coordinates timing and patterns for smooth and agile subconscious movements
Brainstem: base of the cerebrum and anterior to the cerebellum
includes the medulla oblongata, midbrain, and pons
relays info between rest of the brain and the spinal cord
coordinates a lot of automatic functions like respiration, circulation, body temperature, sleep, digestion, and swallowing
Peripheral Nervous System (PNS)
spinal and cranial nerves
communication system between the CNS and the rest of the body
cranial nerves carry impulse to and from the brain
Sensory (afferent) division
sensory nerve fibers
afferent=carry toward (arrive)
receives sensory stimuli to send back to CNS
somatic sensory nerve fibers carry info from the skin, skeletal muscles, and joints
visceral sensory fibers carry info from the visceral organs like lungs, heart, digestive organs, excretory and reproductive organs
sensory nerve receptors take in stimuli from our physical environments and send them to the brain to interpret and respond to
Receptors can be classified by the type of stimulus that activates the receptor
Mechanoreceptors: mechanical force; vibration, pressure, stretch, and touch
Thermoreceptors: change in temperature
Photoreceptors: light
Chemoreceptors: chemicals
Nociceptors: pain
Stimulus > received by receptor > transmission through nerves > spinal cord > brain (all within sensory division
can also trigger action potential (AP) that send signals to the motor division (reflex)
Reflex: automatic reaction to stimuli
Innate (intrinsic): a rapid, predictable motor response to a startling stimulus
Learned (acquired): a response resulting from practice, repetition, or experience
Reflex Arc
reflexes occur over highly specific neural pathways called reflex arcs
5 essential components
Receptor: site of stimulus
Sensory Neuron: transmits impulse from PNS to CNS
Integration Center: “decodes” the signal at a synapse/multiple synapses
Motor Neuron: conducts impulses to an effector organ
Effector: responds by contracting or secreting
Nerve: bundle of axons in PNS
can be classified as cranial (if arises in brain) or spinal (if arises in spinal cord)
there are 31 pairs of spinal nerves in the PNS associated with the spinal cord
there are 12 pairs of cranial nerves in the PNS associated with the brain
they can be sensory (S), motor (M), or both (B)
We all have the same pain threshold to trigger AP in nociceptors, however ALL have a different tolerance for the discomfort that pain produces
Motor (efferent) division
motor nerve fibers
sends out information from the brain to effector organs like muscles (contract) and glands (secrete)
efferent=carrying away (exits)
has two systems: somatic and autonomic
Somatic Nervous System
somatic motor nerve fibers innervate skeletal muscles to control voluntary movements
Somatic nervous system conducts impulses from CNS to skeletal muscles
the neuron’s cell body starts in the CNS and the axon extends all the way to the skeletal muscle it affects
Neurotransmitter acetylcholine (Ach) is released in order to stimulate contractions
Nerve impulses are sent down the neuron’s axon to the neuromuscular function
Ach is released into the synaptic cleft for stimulation
binds to receptors on the skeletal muscle cell which transmits the AP to initiate muscle contraction
Autonomic Nervous System
muscle fibers innervate cardiac and smooth muscles as well as glands to control involuntary movements
conducts impulses from CNS to smooth muscles, cardiac muscles and glands controlling involuntary movements of the heart, lungs, stomach, etc
a 2 neuron chain is used to connect the CNS to effector organs
the 1st neuron’s cell body starts in the CNS and synapses with a 2nd neuron that extends to the effector organ
Neurotransmitter norepinephrine (NE) is releases in the sympathetic nervous system, while Ach is released in the parasympathetic; both can be stimulatory and inhibitory
has two divisions
Parasympathetic Division
“D” division
controls digestion, defecation, and diuresis (urination)
craniosacral nerves (starts at base of brain)
Ganglia are far from the spinal cord, right next to or inside of effector organs
Calm you down; does the opposite of everything the sympathetic does
Parasympathetic the “rest and digest” division; maintains your body and conserves energy for later
set up to communicate to 1 effector organ at a time
preganglionic cells are longer than postganglionic
uses neurotransmitter NE and hormones for stimulation and inhibition
Neurotransmitters: chemicals released from neurons to cross synapse
Hormones: chemicals released from glands into the bloodstream
Sympathetic Division
“E” division
controls exercise, excitement, emergency, and embarrasment
thoracolumbar nerves (start between thoracic and lumbar vertebrae)
Ganglia are in the spinal cord and send signals far distances to effector organs
excites you
Sympathetic: “fight or flight” division
set up so that 1 stress signal to responses in multiple effector organ at once
focuses on what your body needs to do RIGHT NOW
preganglionic cells are shorter that postganglionic
uses neurotransmitter NE and hormones for stimulation and inhibition
is antagonistic to the sympathetic division, but they can work cooperatively
Cranial Nerves of PNS
Olfactory nerve (S): sends scent info from nose to brain
Optic nerve (S): sends visual info from eyes to brain
Oculomotor nerve (M): controls movements of 4 out of 6 eye muscles as well as the pupils’ response to light
Trochlear nerve (M): controls movements for an eye muscle (down and in)
Trigeminal nerve (B): the largest nerve that has three main branches that innervate the face and jaw muscle
Abducens nerve (M): controls movement for an eye muscle (lateral)
Facial nerve (B): operates muscles for most facial expression, taste buds, salivating, blinking, and sends info from outer ear to brain
Vestibulocochlear nerve (S): sends auditory info from cochlea to brain; key for hearing and balance
Glossopharyngeal (B): sensation, taste, swallowing, and sends sensory info from sinuses to brain
Vagus nerve (B): the longest nerve as it extends all the way into the abdomen; mainly controls the heart and digestive tract
Accessory nerve (M): control muscles in neck
Hypoglossal nerve (M) control most muscles in tongue so we can swallow and talk